Preparation process for glyoxal

ABSTRACT

Glyoxal may be produced with a high yield by bringing a gas, which has been formed by diluting ethylene glycol and molecular oxygen with an inert gas, into contact at a high temperature with a silver catalyst in the simultaneous presence of phosphorus or a phosphorus compound so as to effect the gas phase oxidation of the ethylene glycol. 
     In the above process, it is possible to suppress the formation of glycolaldehyde, a reaction intermediate, and also to enhance the stability of the reaction by using silver powder having particle sizes of 1×10 -3  mm or smaller as at least part of the silver catalyst.

TECHNICAL FIELD

This invention relates to a process for preparing glyoxal by the gasphase oxidation of ethylene glycol.

BACKGROUND ART

As preparation process for glyoxal, there have generally been knownprocesses relying upon the oxidation of acetylene or ethylene, theoxidation of acetaldehyde with nitric acid, the oxidation of ethyleneglycol, and the like. However, the process relying upon the oxidation ofacetaldehyde with nitric acid is mainly employed in the industry.

The oxidation of acetaldehyde with nitric acid requires, as an oxidizingagent, nitric acid in an amount at least equal by mole to theacetaldehyde to be reacted. It is thus accompanied by such shortcomingsthat unreacted nitric acid is unavoidably mixed and organic acids areby-produced as impurities in relatively large amounts, thereby making acomplicated separation and purification step indispensable.

On the other hand, a number of proposals have been made for the processfor preparing glyoxal by oxidizing ethylene glycol. There are, forexample, a process for effecting the oxidation with oxygen by using anoxidizing catalyst which is made of copper and/or silver and phosphorus(Japanese Patent Publication No. 1364/1973) and an oxidation processwhich is carried out, in the presence of a catalyst containingphosphorus and copper, phosphorus and silver, or phosphorus, copper andsilver, by incorporating a bromine compound within an amount which doesnot lower the conversion of ethylene glycol to about 90% or less in themixed feed gas (Japanese Patent Laid-open No. 17408/1977). It has alsobeen proposed to use a copper-containing catalyst (U.S. Pat. No.2,339,282) and to effect the oxidation in the simultaneous presence of acopper-containing catalyst and a halogen compound in a cylindricalreactor made of a Cu-Si-Mn alloy. However, the above processes whichemploy these alloy-based catalysts were not significantly advantageousfrom the industrial viewpoint, since the preparations of the catalystswere difficult, their service life in which they can maintain reactionresults at high levels was short, and the regeneration treatments of thecatalysts were complicated.

As processes relying upon the oxidation of ethylene glycol, it has alsobeen proposed to effect the oxidation in the presence of silver crystals

having uniform particle sizes (0.1-2.5 mm)--Japanese Patent Laid-openNo. 103809/1979--and to carry out the oxidation by using acopper-containing catalyst and in the simultaneous presence of aphosphorus compound which is vaporized under reaction conditions(Japanese Patent Laid-open No. 55129/1980). The yields of glyoxal bythese processes were not significantly high and these processes are notsatisfactory as industrial preparation processes.

However, the preparation process for glyoxal relying upon the gas phaseoxidation of ethylene glycol has by itself been considered to be anadvantageous process from the standpoint of economy because it uses, asa raw material source, ethylene glycol which is a derivative of ethyleneoxide obtained from inexpensive natural gas as its starting raw materialand is thus economically superior compared with the oxidation ofacetaldehyde with nitric acid.

DISCLOSURE OF THE INVENTION

An object of this invention is to provide a process for preparingglyoxal with a high yield by the gas phase oxidation of ethylene glycol.

Another object of this invention is to provide a preparation process forglyoxal, which features very little formation of glycol aldehyde as areaction intermediate, is easy to separate and purify the intendedproduct and is thus suitable as an industrial preparation process.

The above and other objects can be achieved by the present inventionwhich will next be defined. Namely, the present invention relates to aprocess for preparing glyoxal by subjecting ethylene glycol to gas-phaseoxidation, which process comprises bringing the ethylene glycol and agas containing molecular oxygen into contact at a high temperature witha silver catalyst in the simultaneous presence of phosphorus or aphosphorus compound to effect the oxidation of the ethylene glycol.

The formation of glycol aldehyde as a reaction intermediate can besharply lowered and the stability of the reaction can be improvedespecially by using, as at least part of the above-described silvercatalyst, fine silver powder having particle sizes of 1×10⁻³ mm orsmaller.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the phosphorus or phosphorus compound may beprovided for the reaction by mixing a predetermined amount of phosphorusor a phosphorus compound in ethylene glycol in advance. Alternatively,it may be added to the reaction system either as is or as a solution,separately from the ethylene glycol. As phosphorus compounds effectivelyuseful in the present invention, may be mentioned inorganic phosphoruscompounds such as ammonium orthophosphate, diammonium hydrogenphosphate,ammonium dihydrogenorthophosphate and ammonium dihydrogenphosphite aswell as a variety of organic phosphorus compounds including primary totertiary phosphines such as mono-, di- and trimethylphosphines,phosphorous esters such as methyl phosphite and ethyl phosphite,dimethyl methylphosphonate, and diethyl ethylphosphonate. However, it isnot preferred to use a phosphorus compound having a high boiling pointbecause it necessitates raising the temperature of an evaporator to aparticularly high level, or it remains in the evaporator and builds upthere in decomposed forms, thereby causing the apparatus material toundergo corrosion and permitting the resultant iron rust and the like toreach the reaction layer so that the reaction may be affecteddeleteriously. More preferably, an organic phosphorus compound having arelatively low boiling point such as, for example, methyl phosphite,ethyl phosphite, methyl phosphate or ethyl phosphate is usedaccordingly.

It is suitable to add the phosphorus or a phosphorus compound in anamount in the range of 1-50 ppm as calculated in terms of phosphorusbased on the ethylene glycol. The addition of phosphorus or a phosphoruscompound suppresses to a significant extent the formation of oxidationproducts such as carbon monoxide and carbon dioxide and decompositionproducts such as formaldehyde and considerably improves the yield ofglyoxal, the intended product. The proportion of produced carbonmonoxide, carbon dioxide or formaldehyde increases immediately even ifthe addition of phosphorus or a phosphorus compound is briefly stopped.Thus, this indicates that the thus-added phosphorus or phosphoruscompound is not acting in a state deposited on the catalyst but isacting effectively in the gas phase.

When the phosphorus or a phosphorus compound is added in any amount inexcess of 50 ppm as calculated in terms of phosphorus, the amount ofproduced glycolaldehyde which is the reaction intermediate increasesand, in addition, the conversion of ethylene glycol decreases and moreunreacted ethylene glycol remains. Therefore, it is not preferred to addthe phosphorus or a phosphorus compound in such amounts. However, theeffects of the phosphorus or a phosphorus compound cannot be drawn outto any sufficient extent and the objects of the present invention cannotbe fulfilled, if the phosphorus or a phosphorus compound is incorporatedin any amounts lower than 1 ppm. As retarding agents against such asilver-base catalytic system, the above referred-to Japanese PatentLaid-open No. 103809/1979 discloses by way of examples halogenatedhydrocarbons, halogens and hydrogen halides. Compared with suchretarding agents, the effects which have been brought about by theaddition of phosphorus or a phosphorus compound in accordance with thepresent invention are remarkable. In view of the still-improved highyield of glyoxal, the process of this invention is very meritorious asan industrial preparation process.

Turning next to the catalyst, it is feasible to use silver of any typeregardless of its preparation process or method, including, for example,silver crystals obtained by the electrolysis method and having particlesizes of 0.1-2.5 mm; fine silver powder

having particle sizes of 1×10⁻³ mm and smaller, for example, fine silverpowder produced chemically, e.g., by the alkaline precipitation method;fine silver powder obtained in accordance with the so-called gasevaporation method, namely, by conducting a variety of heating methodsin an inert gas; or fine silver powder obtained by the vacuumevaporation method; or the like. In the case of a fine powdery silvercatalyst, it may be used solely or in combination with crystallinesilver grains having grain sizes of 0.1 mm or larger.

The process of this invention can be conducted by passing a gas, whichcontains the raw materials, i.e., ethylene glycol and oxygen, as adownward current through a layer of the silver catalyst. Where a silvercatalyst having relatively large grain sizes is used to form the layerof silver catalyst, it is preferred to pack the silver catalyst in atleast three layers in accordance with grain size. It is preferred, forexample, to change the grain size distribution in such a way that thesilver grains become coarser in the direction from the top to thebottom, for example, by packing 10-30 wt. % of silver grains havinggrain sizes of 0.1-0.35 mm in the uppermost layer, 35-65 wt. % of silvergrains having grain sizes of 0.35-0.85 in the next layer and 5-55 wt. %of silver grains having grain sizes of 0.85-2.5 mm in the lowermostlayer. The number of layers may, of course, be increased further. Wherefine silver powder having particle sizes of 1×10⁻³ mm and smaller isused as the silver catalyst and the reaction gas flows, for example, asa downward current through the reactor, it may be mentioned as oneexample of a practical application method to lay crystalline silverparticles having particle sizes of about 0.1 mm and larger over acopper-made wire net of a suitable mesh and then to pack a fine powderysilver catalyst having particle sizes of 1×10⁻³ mm and smaller on thecrystalline silver particles so that the fine powdery silver catalystmay be protected from the scattering loss. It is also feasible to use,in place of silver grains having grain sizes of about 0.1 mm andgreater, an inert carrier having a small specific surface area such asα-alumina or steatite balls. However, such an inert carrier brings aboutsomewhat poorer reaction results compared with silver grains orparticles. This can be attributed to the good thermal conductivity ofsilver. A copper-made wire net is used as mentioned above but it is notfeasible to use a wire net made of iron or the like which serves as acatalyst poison. Needless to say, silver particles or grains havingdifferent particle or grain sizes may also be used as long as they arecombined with at least fine silver powder having particle sizes of1×10⁻³ mm and smaller.

The combined use of the trace amount of phosphorus or phosphoruscompound and the fine powdery silver catalyst, especially havingparticle sizes of 1×10⁻³ mm and smaller, has synergistically combinedthe meritorious features of both phosphorus or a phosphorus compound anda silver catalyst and has hence brought about outstanding effects,although the merits and demerits of the action exhibited by thephosphorus or phosphorus compound per se and those shown by the finepowdery silver catalyst per se are mutually contradictory.

In other words, when the amount of the phosphorus compound is increasedwithout employing the fine powdery silver catalyst, the amounts ofproduced carbon dioxide, carbon monoxide and formaldehyde decrease andthe glyoxal selectivity is improved but glycolaldehyde, which is thereaction intermediate, and unreacted ethylene glycol increase. In theabsence of phosphorus or a phosphorus compound, on the other hand, thereaction results vary depending on the particle or grain sizes of thesilver catalyst to be used. Where the above-mentioned fine silver powderhaving particle sizes of 1×10⁻³ mm and smaller are used as at least partof the catalyst to effect the gas phase oxidation of ethylene glycolinstead of using silver crystals having grain sizes of about 0.1 mm andgreater as the catalyst, the silver powder has remarkably high oxidationactivity but is accompanied by a greater occurrence of decomposition andoxidation products such as carbon monoxide and carbon dioxide.Therefore, the silver powder cannot make the yield of the intendedcompound, glyoxal, significantly better than the silver crystals.

In the presence of both phosphorus or a phosphorus compound and a finepowdery silver catalyst as opposed to the above description, theincrement of glycolaldehyde and unreacted ethylene glycol isconsiderably suppressed and the glyoxal selectivity is improved.

In the sole presence of a crystalline silver catalyst having grain sizesof about 0.1 mm and greater, it is preferred, as described above, thatthe phosphorus or a phorphorus compound be present in an amount of 1-50ppm as calculated in terms of phosphorus based on the ethylene glycol.As the amount of the phosphorus or a phosphorus compound increases, theamounts of decomposition and oxidation products such as carbon dioxide,carbon monoxide and formaldehyde decrease and the glyoxal selectivity isimproved but the unreacted ethylene glycol and glycolaldehyde, thereaction intermediate, tend to increase. In addition, the reactiondeactivating phenomenon is induced to occur and the continuity andstability of the reaction are liable to be impaired due to a reductionin reaction temperature. Where fine silver powder having particle sizesof 1×10⁻³ mm and smaller is used as at least part of the catalyst, itespecially improves the reaction stability to a significant extent andremarkably suppresses the occurrence of unreacted ethylene glycol andglycolaldehyde which is the reaction intermediate. Even if a catalyst ofthe above particle sizes is used, the addition of phosphorus or aphosphorus compound in an amount exceeding 50 ppm as calculated in termsof phosphorus induces a variety of reaction-retarding actions to occurto such an extent that the reaction may not be continued any further.Thus, it is not practical to use phosphorus or a phosphorus compound insuch an excess amount.

In the process of this invention, it is preferred to use the molecularoxygen at a proportion of 0.7-2.0 moles per mole of ethylene glycol. Ifoxygen is present in excess of 2 moles, oxidation products such ascarbon dioxide and carbon monoxide and a decomposition product such asformaldehyde are remarkably increased and the yield of glyoxal islowered. If oxygen is present in any amount less than 0.7 mole, theabove oxidation products are decreased but more unreacted ethyleneglycol is caused to remain. Thus, it is not preferred economically touse oxygen at such a low level. Furthermore, such a low oxygen levelleads to a reduction in reaction temperature and makes it difficult forthe stable reaction to proceed further. Therefore, it is not practicalto use oxygen at such a low level. As molecular oxygen, either pureoxygen or air may be used. The latter is, however, preferred from thestandpoint of economy.

In the present invention, it is preferred to conduct the reaction bydiluting ethylene glycol and molecular oxygen with an inert gas so thatglyoxal can be obtained with a high yield. As the inert gas, as wellknown in the art, may be employed nitrogen, a rare gas such as helium orargon, carbon dioxide or steam. The exhaust gas of the reaction may alsobe recirculated for use as the diluting gas. When effecting the dilutionwith the inert gas, at least 5 moles of the latter gas may be mixed witheach mole of ethylene glycol. If the inert gas is used in any amountless than 5 moles, oxidation products such as carbon dioxide and carbonmonoxide are increased and the yield of glyoxal is thus decreased.

The gas phase oxidation in the process of this invention is suitablycarried out at a reaction temperature in the range of 450°-650° C. Anyreaction temperature lower than 450° C. results in an excessively lowconversion of ethylene glycol, thereby necessitating separating andrecovering a great deal of unreacted ethylene glycol and recirculatingit. Such excessively low reaction temperatures also lead to a greateroccurrence of glycolaldehyde. If the reaction temperature exceeds 650°C. on the other hand, more carbon dioxide and carbon monoxide areproduced as oxidation products. Accordingly, such an excessively highreaction temperature is not preferred.

It is preferred that the residence time of the reaction gas in thesilver catalyst layer not exceed 0.03 second. If the residence time islonger than the aforementioned time period, oxidation products such ascarbon dioxide and carbon monoxide and decomposition products such asformaldehyde are increased.

It is necessary to cool the reaction product gas as soon as possibleafter it has passed out of the catalyst layer. It is not preferred toallow the reaction gas to stay for a long time period in thehigh-temperature zone because the decomposition of glyoxal and the likeare promoted. The thus-cooled gas is subjected to partial condensationas needed, in order to recover unreacted ethylene glycol. Where thereaction gas does not contain unreacted ethylene glycol or containsethylene glycol at such trace concentrations that it does not cause anyproblem in the final glyoxal and it is thus unnecessary to go to thetrouble of removing ethylene glycol from the reaction gas, the reactiongas is cooled and condensed in a heat exchanger and the resultantglyoxal is then separated from the remaining gaseous components inaccordance with the usual absorption procedure which makes use of water.

The thus-obtained aqueous glyoxal solution contains organic acids and atrace amount of formaldehyde as impurities. The formaldehyde is readilyremoved by the usual stripping operation in which steam is blown in theaqueous glyoxal solution. In the course of the stripping operation,parts of organic acids having low boiling points such as formic acid andacetic acid are also removed. An aqueous glyoxal solution obtained bythe oxidation of ethylene glycol in the presence of a fine powderysilver catalyst hardly contains the reaction intermediate,glycolaldehyde, at all compared with that synthesized without the finepowdery silver catalyst. In addition, an aqueous glyoxal solution whichhas been subjected to the formaldehyde stripping treatment containsorganic acids at such a total acid concentration that it is suitable tomaintain the stability of the product. Accordingly, it is unnecessary tosubject the aqueous glyoxal solution to any further separationtreatment. It is thus sufficient to apply only a decolorizationtreatment and/or a treatment with a cation exchange resin as needed.

In this regard, the gaseous components which have been removed beforemay be exhausted as an exhaust gas or may partially be recirculated asan inert gas for reutilization.

In the process of the present invention, it is possible as mentionedabove to prepare glyoxal with a high yield from ethylene glycol whilesuppressing the occurrence of by-products. In addition, the process ofthis invention features the excellent stability of the gas phaseoxidation. Furthermore, the process of this invention has a furthermerit that the separation and purification steps of the reaction productcan be simplified.

The present invention will hereinafter be described in further detail bythe following Examples:

COMPARATIVE EXAMPLE 1

Seventeen g of silver grains obtained by the electrolysis of an aqueoussilver nitrate solution and having grain sizes of 0.84-1.5 mm were laidin the lowermost layer of a reactor, followed by 10 g of silver grainsobtained in the same manner and having grain sizes of 0.35-0.84 mm overthe first-mentioned silver grains, 8 g of silver grains obtained in thesame manner and having grain sizes of 0.16-0.35 mm over thesecond-mentioned silver grains, and 1.0 g of fine powdery silverobtained by the vacuum deposition method and having an average particlesize of about 7×10⁻⁵ mm as the uppermost layer. The overall height ofthe packed layer was about 30 mm.

Into the thus-prepared reactor, ethylene glycol, steam, air and nitrogenwere charged through a preheater at 162 g/hr, 162 g/hr, 280 liters/hrand. 800 liters/hr, respectively, as a downward current. They werereacted at a reaction temperature of 521° C. After cooling the reactiongas, reaction products were separated and collected in an absorptiontower which employed water as its absorbent.

As a result, the conversion of ethylene glycol was 100% and the glyoxalselectivity and formaldehyde selectivity were 44.3% and 12.4%,respectively.

EXAMPLE 1

A reaction was carried out in the same reactor and under the sameconditions as those used in Comparative Example 1 except that 26.8 ppmof triethyl phosphite (5 ppm as calculated in terms of phosphorus),based on the feed ethylene glycol, was added. The reaction temperaturewas lowered to 502° C.

The reaction resulted in a conversion of ethylene glycol of 100%, aglyoxal selectivity of 80.1% and a formaldehyde selectivity of 2.1%.Glycolaldehyde, which is the reaction intermediate, was produced in atrace amount.

COMPARATIVE EXAMPLE 2

Thirty-eight grams of a granular silver catalyst, which had beenobtained by the electrolysis of an aqueous silver nitrate solution, werepacked in a reactor by first laying 20 g of silver grains having grainsizes of 0.84-1.5 mm and obtained by classification as a lowermostlayer, then 10 g of silver grains having grain sizes of 0.35-0.84 mm andobtained in the same manner over the first-mentioned silver grains, andfinally 8 g of silver grains having grain sizes of 0.16-0.35 mm andobtained in the same manner as an uppermost layer. The overall height ofthe packed layer was about 30 mm.

A feed gas having the same composition as that of Comparative Example 1was caused to pass through the above-prepared reactor without theaddition of triethyl phosphite under the same conditions as inComparative Example 1 except that the reaction temperature was set at510° C. The conversion of ethylene glycol, glyoxal selectivity andformaldehyde selectivity were 100%, 53% and 5.2%, respectively.

EXAMPLE 2

A reaction was carried out under the same conditions as in ComparativeExample 2 except that 26.8 ppm of triethyl phosphite (5 ppm ascalculated in terms of phosphorus), based on the ethylene glycol, wasadded and the reaction temperature was set at 501° C.

The conversion of ethylene glycol, glyoxal selectivity and formladehydeselectively were 100%, 80.4% and 2.5%, respectively, but the reactionintermediate, glycolaldehyde, was also by-produced at a rate of 1.3% interms of selectivity.

COMPARATIVE EXAMPLE 3

In the reaction of Example 2, the addition of triethyl phosphite wasstopped and, upon an elapsed time of 10 minutes after the stoppage ofthe triethyl phosphite addition, reaction products were separated andcollected and then subjected to analysis. The reaction temperature wentup to 509° C. The conversion of ethylene glycol, glyoxal selectivity andformaldehyde selectivity were 100%, 53.5% and 5.4%,respectively. Theformation of glycolaldehyde was not observed.

EXAMPLE 3

A reaction was carried out at 496° C. under the same reaction conditionsas in Comparative Example 2 except that 53.6 ppm of triethyl phosphite(10 ppm calculated in terms of phosphorus), based on the ethyleneglycol, was added.

The conversion of ethylene glycol, glyoxal selectivity and formaldehydeselectivity were 98.9% , 84.6% and 1.0%, respectively. It was observedthat the yield of glyoxal was improved owing to the increment in theadded phosphorus compound. On the other hand, it was also found that thereaction temperature tended to vary as the feeding rates of the rawmaterials changed.

EXAMPLE 4

A reaction was carried out under the same reaction conditions as inExample 1 except that 53.6 ppm of triethyl phosphite (10 ppm ascalculated in terms of phosphorus), based on the ethylene glycol, wasadded and the reaction temperature was set at 497° C.

The conversion of ethylene glycol, glyoxal selectivity and formaldehydeselectivity were 99.9%, 84.1% and 0.9% respectively. This reactionresult exhibits the improved effects of the combined addition of thefine powdery silver catalyst and phosphorus compound in that there islittle formation of by-products and the selectivity of the intendedproduct, glyoxal, is extremely high. In the operation performance, thereaction temperature did not vary and excellent stability was observed,unlike Example 3.

EXAMPLE 5

Thirty-nine grams of silver nitrate were dissolved in 140 ml of purewater, followed by a gradual dropwise addition with stirring of anaqueous solution which had been prepared by dissolving 20 g of NaOH in30 ml of pure water. The resultant brownish precipitate was collected bysuction filtration and washed well with pure water. The precipitate wasthen suspended in 180 ml of pure water, to which 12 ml of a 30% aqueoussolution of formaldehyde was added while stirring the suspensionvigorously. At the same time, an aqueous 5-N NaOH solution was added tomaintain the pH of the suspension within 8-12. The resultant mixture wasthen stirred for 30 minutes and then allowed to stand. The thus-formedgray precipitate was collected by suction filtration and washedthoroughly with pure water. It was then dried at 110° C. Twenty-fivegrams of silver having an average particle size of about 0.25 μm (2500Å) were obtained.

In a reactor, 17.5 g of silver grains similar to those employed inExample 1, obtained by the electrolysis of an aqueous silver nitratesolution and having grain sizes of 0.84-1.5 mm, was spread as alowermost layer. Laid successively over the lowermost layer were 10 g ofsilver grains obtained in the same manner and having grain sizes of0.35-0.84 mm, 8 g of silver grains obtained in the same manner andhaving grain sizes of 0.16-0.35 mm, and finally 1.0 g of the fine silverpowder obtained above in accordance with the alkaline precipitationmethod. The overall height of the packed layer was about 30 mm.

A reaction was conducted at 501° C. in the same manner as in Example 1,by adding 26.8 ppm of triethyl phosphite (5 ppm as calculated in termsof phosphorus) based on the ethylene glycol. As a result, the conversionof ethylene glycol, glyoxal selectivity and formaldehyde selectivitywere 100%, 81.0% and 2.3%, respectively, and the occurrence of thereaction intermediate, glycolaldehyde, was limited to a trace amount.

COMPARATIVE EXAMPLE 4

A reaction was carried out in the same manner as in Example 2 exceptthat 50 ppm of ethylene dichloride (35.8 ppm as chlorine), based on theethylene glycol, was added instead of triethyl phosphite and thereaction temperature was set at 498° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 100%, 60.8% and 5.6%, respectively.

COMPARATIVE EXAMPLE 5

A reaction was carried out in the same manner as in Example 2 exceptthat 50 ppm of bromoform (47 ppm as bromine), based on the ethyleneglycol, was added in place of triethyl phosphite and the reactiontemperature was set at 505° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 100%, 62.1% and 6.3%, respectively.

EXAMPLE 6

A reaction was effected in the same manner as in Example 2 except that22.7 ppm of trimethyl phosphate (5 ppm as calculated in terms ofphosphorus), based on the ethylene glycol, was added and the reactiontemperature was set at 497° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 99.0%, 78.6% and 2.3%, respectively.

EXAMPLE 7

A reaction was carried out in the same manner as in Example 2 exceptthat 36.2 ppm of (C₂ H₅ O)₂ POCH₂ COOC₂ H₅ (5 ppm as calculated in termsof phosphorus), based on the ethylene glycol, was added and the reactiontemperature was set at 500° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 99.9%, 81.0% and 3.2%, respectively.

EXAMPLE 8

A reaction was carried out in the same manner as in Example 2 exceptthat 12.8 ppm of diammonium hydrogenphosphate (NH₄)₂ HPO₄ (3 ppm ascalculated in terms of phosphorus), based on the ethylene glycol, wasadded and the reaction temperature was set at 505° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 100%, 70.2% and 2.9%, respectively.

EXAMPLE 9

A reaction was effected in the same manner as in Example 2 except that19.1 ppm of triethylphosphine (5 ppm as calculated in terms ofphosphorus), based on the ethylene glycol, was added and the reactiontemperature was set at 505° C.

As a result, the conversion of ethylene glycol, glyoxal selectivity andglycolaldehyde selectivity were 100%, 70.7% and 3.3%, respectively.

We claim:
 1. A process for preparing glyoxal by subjecting ethyleneglycol to gas-phase oxidation, which process comprises bringing theethylene glycol and a gas containing molecular oxygen into contact at atemperature in the range of 450°-650° C. with a silver catalyst in thesimultaneous feeding of one or more phosphorus compounds in a vaporphase to effect the oxidation of the ethylene glycol, the amount of themolecular oxygen used is 0.7-2.0 moles per mole of the ethylene glycoland the amount of the fed phosphorus compounds is 50 ppm or less ascalculated in terms of phosphorus relative to the ethylene glycol.
 2. Aprocess according to claim 1, wherein the silver catalyst has particlesizes of about 2.5 mm or smaller.
 3. A process according to claim 1,wherein at least part of the silver catalyst is fine silver powderhaving particle sizes of 1×10⁻³ mm or smaller.
 4. A process according toclaim 1, wherein the phosphorus compounds are fed in an amount of 1-50ppm as calculated in terms of phosphorus relative to the ethyleneglycol.
 5. A process according to claim 1, wherein the phosphoruscompound is one or more phosphorus compounds selected from the groupconsisting of methyl phosphite, ethyl phosphite, methyl phosphate andethyl phosphate.
 6. A process according to claim 1, wherein 5 or moremoles of a diluting inert gas are used per mole of the ethylene glycol.7. A process according to claim 1, wherein the residence time of thereaction gas in the layer of the silver catalyst is controlled to 0.03second or less.
 8. A process for preparing glyoxal comprising:(a)feeding simultaneously in vapor phase to a reactor;(i) ethylene glycol;(ii) a gas, said gas containing molecular oxygen between 0.7 and 2.0moles per mole of said ethylene glycol; and (iii) at least onephosphorus compound said phosphorus compound being fed in said vaporphase at 50 ppm relative to said ethylene glycol; and (b) contactingsaid ethylene glycol, said gas, and said phosphorus compound with apowdered silver catalyst at a temperature between 450° C. and 650° C.,said powder silver catalyst having particle sizes of up to 1×10⁻³ mm,whereby said ethylene glycol and said oxygen are converted to glyoxal.